One example of a method for treating organic wastewater such as sewage involves removing the raw sludge (primary sludge) from the organic wastewater followed by a biological treatment such as an activated sludge treatment, thereafter removing excess sludge from the biologically treated organic wastewater while the solution being discharged, the raw sludge and the excess sludge is mixed and subjected to methane fermentation treatment, so as to reduce the volume of the sludge.
Due to containing more easily degradable organic matter than excess sludge does, raw sludge is more adaptable to methane fermentation in aspects of digestive efficiency, amount of digestion gas generated, digestion speed, etc. able to yield a higher energy recovery effect. Conversely, excess sludge is a kind of substrate rich in hard-to-degrade cellulose and proteins derived from such as microbial cells, which requires time to get decomposed. Therefore it has a lower energy recovery effect than raw sludge does. Because of that when a mixture of raw sludge and excess sludge is subjected to methane fermentation, the performance of methane fermentation gets worse, resulting in poor energy recovery efficiency and larger methane fermentation tanks in order to obtain long hydraulic retention time.
In order to improve the property of such excess sludge, technologies for example solubilization treatment were developed. Patent document 1 discloses a method for treating organic wastewater, which includes a primary sludge removal step, an activated sludge treatment step, an excess sludge removal step, and a methane fermentation treatment step with the primary sludge and the excess sludge, where the excess sludge being subjected to a solubilization treatment at 60-90° C., heat from the solubilization-treated excess sludge was utilized to heat the primary sludge, the heated primary sludge was subjected to an acid fermentation treatment for 3-4 days at 30-40° C., and then mixed with the solubilization-treated excess sludge for methane fermentation treatment.